All,

We already have a great thread on this topic here. TomZ has also shared some nice info related to the use of screws with 3D printed designs here and here.

The best summary is offered in Oskar's post here:However I have a question that I haven't seen answered and I didn't want to bump that old thread so I've started this one. If a mod feels this should be in that thread please feel free to merge these.

In the case of a pin-in-hole constructions a 0.2mm gap is added to assure the pin can turn in the hole. However what is the best tolerance to use if the pieces are intended to be locked together and not rotate relative to each other? Say you have a square peg you want to fit tight into a square hole. I'm thinking I want to use something less then 0.2mm but 0mm may cause problems with even getting the peg into the hole. I'm going to guess I should use 0.05mm or 0.1mm but I wanted to hear from the experts to see if I could avoid some expensive trial and error.

Thanks,
Carl

The tolerances provided in Oskar's post are a bit on the high side. 0.3 and 0.15 or 0.1 respectively will do.

What about 0.2 and then use some super glue to hold it in? Trying to get it perfectly snug is troublesome, as you will be subjected to the randomness of printing: some will fit, others won't.
Or if you don't want to use superglue, it is possible to design some hooks that create a snap fit. I've done that on one of my Constrained Cube designs and also on the Multidodecahedron.

The "snap" mechanism for the caps on the Multidodecahedron works incredibly well.

I think something like that could be adapted for the 8 tips of the core of the Doctor Skewb so that the corner arms can be screwed into the core.

Just to be perfectly clear, you are saying 0.3mm will do where Oskar/Eric state "3: All gaps that are pulled open need 0.4 mm clearance." And you are saying 0.15mm or 0.1mm will do where Oskar/Eric state "5: Pin-in-hole constructions need 0.2 mm gap at all side to assure smooth sliding/turning." Correct?I'm working on the challenge Oskar gave me and the connection point I'm thinking of is the face center to the geared axis under it. I don't want to glue these together as this will be a kit designed to be taken apart and reassembled as a different puzzle. As for holding them together that isn't a problem. The face center will be screwed into the core and both the face center and the axis its connected to will rotate around the arm off the core that the screw is screwed into. Its just that as the face center drives the axis (or the other way around depending on which side of the puzzle is being turned) so I want to minimize slop. So what I'm after is the tightest tolerance I can use yet still be certain the piece will fit. If you are saying 0.1mm is enough to allow turning (case 5 above), I'm going to guess that 0.05mm may be enough in this case.

Thanks,
Carl

Yes, I may revisit the Doctor Skewb down the road. However I have several other puzzles which I want to make "real" before I take a shot at trying to make one of the already "real" ones even better.

Carl

Yes, those interpretation of my measurements is correct.

I took a look at some of my designs. If you want a nicely rotating axle in a hole 0.1mm applied to the radius will do (so a 2.8mm diameter axle in a 3mm hole).
For a square peg in a square hole, 0.1mm is the measurement to use as well. On a single print, a square peg of 2.8mm will sometimes slide in to a 3mm square hole easily and sometimes it will require quite some force to get it going. I think if you go down to 0.05mm it will not work well.
Admittedly, I have used 0.05mm (15.9mm square peg in 16mm hole) but I think that only really worked because the square was really large and the outer walls thin so they had room to flex out. Though, the fit is rather loose so perhaps it will work, even with stronger geometry.

But then, you can only get so far by having super tight tolerances. Even on a design where the center axle is one piece there is still considerable slack.